Conventionally, a cooker, or a cooking device, generally has a pot member adapted to be placed on a furnace or a heater for transferring heat from the heat source to foodstuff contained in the pot member, until the foodstuff is cooked. A cooker provided with a built-in heater is convenient and has become very popular; such a cooker with a built-in heater generally has an inner pot made of thermally conductive material, and an outer pot having a thermally insulated casting, wherein the inner pot is removably placed in the outer pot which is provided with a built-in heater and a control device. In use, foodstuff is placed in the inner pot, which is then placed in the outer pot, and then the built-in heater of the outer pot is turned on to supply heat to the foodstuff in the inner pot. Some conventional electric cookers are provided with a timer or automatic temperature control device to enable users to set an appropriate cooking time. However, even with a sophisticated automatic control device, it is often difficult for an unskilled housewife to cook a foodstuff appropriately with a conventional electric cooker without consulting a cooking manual.
There is a special type of an electric cooker known as a "slow cooker" which is designed to cook foodstuff slowly with a minimum supply of heat (or energy) throughout the cooking process. However, it has been found out by the inventor of this invention that food can be better and easier cooked by initially supplying an ample amount of heat (or energy) for boiling for a short period of time, and then let the semi-cooked foodstuff cooked by its own latent heat thereafter without a continuous supply of heat energy.